Ophinia. Long time no see.

Anyway, Ophinia gets an interesting role in Undertale, functionally being a 'surprise' final challenge, though er exact purpose is slightly different.
This form is her Angel of Death encounter, a replacement for Sans, where she hopes to squeeze a single spare out of you to prove you're not a monster, just 'very, VERY rude'. This goes as far as to offer the choice without it affecting the ending, hoping that's all the player REALLY cares about: "Listen... spare me now, nothing changes. Those dudes will die, you'll see your darkest truth, bla bla. Nothing changes... except you prove you're not heartless. That you're not a robot following orders like a tool. JUST ONE. No consequences. I'll die anyway once you pull out your weapon... it's just that you'll prove to me and the world... that you're not very, VERY rude. So... all kills... just a peace of mind. Take your pick, slayer."

Her AOD powers are borrowed from the 'Real World', since the UNdying is stuck here so long he lost his ability to overcome the player. She loses these, as she 'integrates'. The use of the royal symbol appears to be some sort of anchor that lets her cheat this way, if only temporarily. During attacks, her regular halberd appears, implying her lack of clothes is entirely involuntary (though conscious).

If met at the end, she only wants 'mercy'. Spare her, you get a SLIGHTLY better ending (like, Asgore/Flowey says you spared at least one, but otherwise you're 100% boned). However, if you fight her to the end, you get a spare/kill option. This has NO effect, other than that she becomes Angry Ophinia if met at her hiding spot if you killed her.
Similar to Sans, her Angel of Death (final room) encounter is a pure marathon, as she cannot be harmed (and dodges sometimes for fun). After 10 turns, she becomes slower. After 15, even slower. At 20, she MUST be attacked and becomes mortally wounded. Then, she offers a final (ending unchanging) chance to save her. Mostly to ease YOUR mind, as she wants to be sure you're not cold and clinical, going as far as to break the fourth wall to make sure you know it's YOUR mental health you're judging over.

If met at her hiding spot (talk to NPCs near a locked building a LOT, hinted at during a genocide) and you can befriend her in a VERY easy version. After doing this, she explains more about the Undying One and herself, and how her 'fresh' entry makes her immortal, but only until she integrated into 'the box' and becomes mortal like everything else. Not being a monster, however, she leaves a corpse and a soul, neither of which can be killed; only observed (and talked to, be she's mad).
After meeting her there once, she joins all subsequent playthroughs as a recurring character (similar to Genocide-corruption, it is NOT in the normal safe-place). During this, boss fights mostly trigger her.
If you kill Toriel, she'll dig a cenotaph (no corpse, dude) or Toriel, feeling remorse. Unless you're on a Genocide run, she forgive you, saying you didn't know better (if you tried mercy at least once) or should be more gentle. If Toriel lives, the player can listen at the Ruins exit to hear her consoling Toriel, insisting 'he'll come visit, and his heart is with you' (pacifist) or that 'he'll be fine, you'll see, have faith' (neutral).
During a genocide run, any killed boss' cenotaph is marked with a blood-red handprint, and Ophinia refuses to talk. At Toriel's grave, she swats the player aside if the 'attack' option is selected (only availible on Genocide runs), insisting 'there's only one monster here, and it will learn this kind soul's terror soon enough', refusing to directly acknowledge the player.
Interestingly, she NEVER uses the player's name outside of the first metting, where she throws it aside, instead using 'kid' as she finds the player quite cute.
As she's not a monster (or have a soul, being from outside) she easily pushes the player around when needed, but is unable to use this in the AOD encounter, as this fight 'integrates' her, forcing her to lose to the player's corruption of the world.

Generally, she's nice, but not as forgving as the Undying One, being much faster to criticise the player, but also offers advice how to spare bosses. She is completely award of the player's ability to SAVE (remembering any reloads), thinks Flowey is 'a piece of dung, but hardly as dangerous as you can be' and feels that the Dreemurr situation is 'heartbreaking' which explains why she cares so much for Toriel and even tries to encourage the player to spare Asgore, no matter the cost.
If alive and met, she appears before the Ultra Final Pacifist boss together with the Undying One, insisting the player soothe the soul instead of fight it.

Her relation with the Undying One is friendly, though implied to be VERY professional, as they're both security measures to keep the player from becoming too powerful with madness.

ATTACKS:

• Slashy! Slashes her halberd around, because why not.
• Firestorm! Mimics Toriel's move to the T.
• Trident Trounce! Launches several tridents akin to Asgore's.
• Bone Breaker! A random move involving bones. Why not?

ACT:

• CHECK - INF ATT, INF DEF, a divine being that is functionally immortal. Might want to wait until she runs outta god gas.
• Share - Share a milkshake with her.
• Kiss - Risky, but randomly earns a lot of goodwill!
• Pray - Wonders what you're praying to, since you've clearly lost any faith with all that murder (AOD), wonders if you're praying to her, or that 'hag' Toriel, before giggling and admitting Toriel would find it cutie.
• Mercy - Insists that's not the same as Sparing, which is what she really wants (AOD) or thinks there's no fighting anyway (friendly).